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Redescription of the argyrolagid Microtragulus bolivianus (Metatheria, Polydolopimorphia, Bonapartheriiformes) based on new remains from Northwestern Argentina

M. Judith Babot and Daniel A. García-López

ABSTRACT

This work is based on new dental, cranial, and postcranial remains of the argyro-lagid Microtragulus bolivianus (Metatheria, Polydolopimorphia, Bonapartheriiformes)and dental pieces of Microtragulus sp. coming from late Pliocene-early Pleistocene lev-els of the Uquía Formation exposed at San Roque, Humahuaca (Jujuy Province,Argentina). They were found in association with amphibians, lizards, birds, rodents,and didelphid marsupials, forming an assemblage probably generated by the trophicactivity of owls. Specimens were assigned to M. bolivianus based on the followingcombination of features: M3 subcircular, with a flexus between the mesiolabial lobeand the metacone, paracone and metacone not differentiated in M4, absence of entof-lexid in m1-2 and shallow entoflexid in m3, proportionally large talonid in m4, with a dis-tinguishable distal flexid. Microtragulus bolivianus, initially recorded in Pliocenesediments exposed at the Bolivian Altiplano, was represented only by a mandibularfragment with one incisor and m3-m4. Based on a much more complete sample wepresent a detailed dental and postcranial description of the species. Since 1904 thefamily name Argyrolagidae and the generic names Microtragulus and Argyrolagushave been subject of several nomenclatural changes. A revision of these modifica-tions, mainly those occurring in the last 40 years, is also presented. Furthermore, weanalyze some mandibular traits of argyrolagids such as the maxillary canal (retrodentalcanal), a very odd feature present in all the members of the family, which could berelated to the passage of a connection between the inferior alveolar and inferior orbitalveins, as in some extant mammals.

M. Judith Babot. Fundación Miguel Lillo, Miguel Lillo 251, 4000, San Miguel de Tucumán, Tucumán, Argentina; [email protected] CONICET-Consejo Nacional de Investigaciones Científicas y Técnicas, ArgentinaDaniel A. García-López. CONICET-Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina, Facultad de Ciencias Naturales, Miguel Lillo 205, 4000, San Miguel de Tucumán, Tucumán, Argentina; [email protected], and Instituto Superior de Correlación Geológica-INSUGEO (CONICET)

Babot, M. Judith and García-López, Daniel A. 2016. Redescription of the argyrolagid Microtragulus bolivianus (Metatheria, Polydolopimorphia, Bonapartheriiformes) based on new remains from Northwestern Argentina. Palaeontologia Electronica 19.2.23A: 1-23palaeo-electronica.org/content/2016/1481-argyrolagid-from-northwestern-argentina

Copyright: © July 2016 Society of Vertebrate Paleontology. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.creativecommons.org/licenses/by/4.0/

BABOT & GARCÍA-LÓPEZ: ARGYROLAGID FROM NORTHWESTERN ARGENTINA

Keywords: Argyrolagidae; Bonapartheriiformes; Marsupialia; Uquía Formation; Umala Formation;Marplatan SALMA

Submission: 3 August 2015 Acceptance: 2 May 2016

INTRODUCTION

Argyrolagidae is an odd group of small fossilmetatherians, which inhabited South Americaduring the Cenozoic. Its temporal range extendsfrom the late Oligocene (Deseadan South Ameri-can Land Mammal Age [SALMA]) to the earlyPleistocene (Marplatan SALMA, Sanandresiansubage). The family is only known from Argentinaand Bolivia, although the closely related argyro-lagoid Khlonia is also present in Chile (Ameghino,1904; Kraglievich, 1931; Simpson, 1970a, 1970b;Hoffstetter and Villarroel, 1974; Wolff, 1984; Villar-roel and Marshall, 1988; Goin et al., 2000, 2010;Carlini et al., 2007). Some of the most remarkablefeatures of the group are the presence of an elon-gated snout, large orbits, very large palatal vacu-ities and wide incisive foramina, inflated tympanicbullae, procumbent lower incisors, hypsodont andhighly modified molars, short, and high mandibularbody, low and short coronoid process, mandibularnotch long, and maxillary canal present (retroden-tal canal sensu Hofftsetter and Villarroel, 1974).The postcranial skeleton also shows several pecu-liarities such as the shortening of the forelimb, thelengthening of the tibia-fibula and metatarsals, andthe reduced number of elements in the posteriorautopodium, specifically the metatarsals and pha-langes, all these features associated with a bipedalstance and leaping locomotor habits (Simpson,1970a).

For more than a century, several authors stud-ied the taxonomy, anatomy, phylogeny, and paleo-biology of the group (Ameghino, 1904, 1906;Rovereto, 1914; Kraglievich, 1931; Rusconi, 1933,1936; Simpson, 1970a, 1970b; Hoffstetter and Vil-larroel, 1974; Wolff, 1984; Villarroel and Marshall,1988; Sánchez-Villagra and Kay, 1997; Goin et al.,2000; Sánchez-Villagra et al., 2000; Sánchez-Villa-gra, 2001; Abello et al., 2002; Carlini et al., 2007;Zimicz, 2011; Goin and Abello, 2013). There arecurrently five genera and 12 recognized species.Two genera are monospecific (Hondalagus Villar-roel and Marshall, 1988 and Anargyrolagus Carlini,Pascual, and Goin, 2007) while ProargyrolagusWolff, 1984, Argyrolagus Ameghino, 1904, andMicrotragulus Ameghino, 1904 are represented bytwo or more species (see Table 1).

Microtragulus is the most widely distributedargyrolagid, including records in central and north-western Argentina and the Bolivian Plateau. Thecurrent geographical record indicates a disjunctdistributional pattern comprising coastal as well ashighland areas. Microtragulus reigi is the only spe-cies known from lowland areas (the existence of M.argentinus remains questionable; see Discussion)and M. catamarcensis and M. bolivianus arerestricted to mid and high altitudes (García-Lópezand Babot, 2015). The oldest record of the genus,a mandibular fragment of M. catamarcensis, isknown from ?early Miocene levels exposed atMendoza Province, Argentina (see Garrido et al.[2014] and García-López and Babot [2015]), andthe more recent records are known from the latePliocene-early Pleistocene (M. bolivianus; Marpla-tan SALMA, probably Vorohuean subage; Ortiz etal., 2012) and early Pleistocene (M. reigi; Marpla-tan SALMA, Sanandresian subage; Cione andTonni, 2005; Cione et al., 2015).

The genus was initially defined based on themorphology of its lower molars (Simpson, 1970a),but additional characters of the upper dentitionhave been recently described for this taxon(García-López and Babot, 2015). The dental fea-tures that define Microtragulus are the presence ofa distinct ectoflexid (labial groove) separating thetrigonid from the talonid and absent or vestigialentoflexid (lingual groove) in m1, trigonid with adominant metaconid, lower ectostylid and protoco-nid pointing mesiolabially and separated from theectostylid by a groove (in m1 and/or m2), reducedtalonid with a distinctive tongue-like hypoconid inm1-2, well-developed entoconid, and hypoconulidabsent or vestigial, and reduced m4, both in lengthand width, bearing a vestigial talonid separatedfrom the trigonid by ectoflexid and entoflexid.Regarding the upper dentition, the distinctive fea-tures include a reduced StE in M1, poorly-devel-oped paracone coalescent with metacone, reducedmesiolabial lobe in M2 and M3, and convex labialoutline, particularly in M2 (García-López andBabot, 2015).

Microtragulus bolivianus is the most northernspecies of the genus. It was found by Hoffstetterand Villarroel (1974) in levels of the Umala Forma-tion exposed at the locality of Viscachani (Bolivia;

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PALAEO-ELECTRONICA.ORG

Pliocene levels dated between 5.3-2.9 Ma [Mar-shall et al., 1992; MacFadden et al., 1994]) anddescribed on the basis of scarce remains (a rightincomplete mandible preserving only one incisorand m3-4). In this contribution, we present newmaterials of M. bolivianus including dental, mandib-ular, and postcranial elements. The materials arepart of a microvertebrate fossil assemblage, proba-bly produced by the activity of predatory birds,recovered in sediments of the Uquía Formationcropping out in the locality of San Roque, 2 kmsouth of Humahuaca town, Jujuy Province, north-western Argentina (Figure 1.1-3). Based on thefossil rodent assemblage Ortiz et al. (2012) sug-gested that xeric palaeoenvironmental conditionswould have prevailed in the region by the end ofthe Pliocene. The presence of the argyrolagid

Microtragulus, for which adaptations to arid envi-ronments were inferred on the basis of their mor-phological similarity with kangaroo rats, wouldreinforce this hypothesis (Ortiz et al., 2012).

In this work we provide detailed anatomicaldescriptions, comparisons, and a revised diagnosisof the species, which was poorly known until now.Additionally, we present an updated revision of thenomenclatural changes suffered by the taxonomicname Argyrolagidae, and examine some particulartraits of the dentary of the group.Institutional abbreviations. GHUNLPam, Facul-tad de Ciencias Exactas y Naturales de la Universi-dad Nacional de La Pampa, La Pampa, Argentina;JUY-P, Museo de Geología, Mineralogía y Paleon-tología, Instituto de Geología y Minería, Universi-dad Nacional de Jujuy, San Salvador de Jujuy,

TABLE 1. List of the genera and species included in the Family Argyrolagidae and their temporal and geographic distri-bution. Abbreviations: AR, Argentina; BO, Bolivia.

Species Temporal distribution Geographic distribution References

Proargyrolagus bolivianus Wolf, 1984

Late Oligocene (Deseadan) Salla-Luribay Basin (BO) Wolf (1984); Sánchez-Villagra and Kay (1997)

Proargyrolagus argentinus Goin and Abello, 2013

Early Miocene (Colhuehuapian)

Puesto Almendra, Chubut (AR)

Goin and Abello (2013)

Anargyrolagus primus Carlini, Pascual, and Goin 2007

Early Miocene (Colhuehuapian)

Lower Chubut River valley, Chubut (AR)

Carlini et al. (2007); Goin and Abello (2013)

Hondalagus altiplanensis Villarroel and Marshall, 1988

Middle Miocene (Laventan) Quebrada Honda (BO) Villarroel and Marshall (1988); Sánchez-Villagra et al. (2000)

Microtragulus argentinus Ameghino, 1904

?Early Pliocene (Montehermosan)

Monte Hermoso, Buenos Aires (AR)

Ameghino (1904); Simpson (1970a); Tomassini et al. (2013)

M. catamarcensis (Kraglievich, 1931)

?Early Miocene - Early Pliocene

25 de Mayo, Mendoza (AR)?Andalhuala, Catamarca (AR)

Kraglievich (1931); Simpson (1970a); Garrido et al. (2014); García-López and Babot (2015)

Microtragulus sp. ?Late Miocene North of Tucumán (AR) García-López and Babot (2015)

M. reigi Simpson, 1970a Middle Pliocene – Early Pleistocene; Chapadmalan - Marplatan

Punta San Andrés, Buenos Aires (AR)

Simpson (1970a); Cione and Tonni (2005)

M. bolivianus Hoffstetter and Villarroel, 1974

?Early Pliocene - Early PleistoceneMontehermosan - Marplatan

Vizcachani (BO)San Roque, Jujuy (AR)

Hoffstetter and Villarroel (1974); Ortiz et al. (2012)

Argyrolagus palmeri Ameghino, 1904

Early Pliocene; Montehermosan

Monte Hermoso, Buenos Aires (AR)

Ameghino (1904); Tomassini et al. (2013)

A. scagliai Simpson, 1970a Middle Pliocene; Chapadmalalan

Miramar, Buenos Aires (AR) Simpson (1970a)

A. parodii Rusconii, 1933 Middle Pliocene; Chapadmalalan

Chapadmalal, Buenos Aires (AR)

Rusconi (1933); Simpson (1970a, b)

A. rusconii (Goin, Montalvo, and Visconti, 2000)

Late Miocene Bajo Giuliani, La Pampa (AR) Goin et al. (2000); García-López and Babot (2015)

Argyrolagus sp. ?Late Miocene Caleufú, La Pampa (AR) Abello et al. (2002)

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Argentina; MACN, Museo Argentino de CienciasNaturales “Bernardino Rivadavia”, Buenos Aires,Argentina; MLP, Museo de La Plata, Buenos Aires,Argentina; MMP, Museo Municipal de CienciasNaturales “Lorenzo Scaglia”, Mar del Plata, Bue-nos Aires, Argentina; MNHN-BOL-V, Museo deHistoria Natural, Colección Paleontología de Verte-brados, La Paz, Bolivia; MPEF-PV, Museo Paleon-tológico “Egidio Feruglio”, Sección Paleontologíade Vertebrados, Trelew, Argentina; PVL, ColecciónPaleontología de Vertebrados Lillo, Instituto MiguelLillo, Tucumán, Argentina.Anatomical abbreviations. M/m, upper/lowermolar; P/p, upper/ lower premolar; St, stylar cusp.

MATERIALS AND METHODS

The nomenclature of molar cusps follows theproposal of Goin et al. (2010) and Goin and Abello(2013) who analyzed the changes in molar mor-phology from Praedens and Klohnia (as basalArgyrolagoidea) to Proargyrolagus and Anargyrola-gus. The arrangement of molar cusps in Micro-tragulus bolivianus (Figure 2) is based on thishypothesis and in García-López and Babot (2015).Cusp names are used as if they were actually pres-ent, although according to the condition of typicaleuhypsont teeth, the cusped surface is quicklyworn away after tooth eruption (sidewall hypsod-onty sensu Koenigswald, 2011). Their relative posi-tion is inferred by the occlusal relief and toothoutline. The designation of the first lower incisor asi1 follows Goin and Abello (2013). Up to now, thereis no supporting evidence for Hershkovitz (1995)

scheme in Argyrolagidae and hence, the identity ofincisors is still under discussion (e.g., Sánchez-Vil-lagra and Kay, 1997; Sánchez-Villagra, 2001).

We assume that in Microtragulus bolivianusthe premolar immediately anterior to the first molaris the third premolar, based on basal members ofthe group that preserve the complete dental series(Proargyrogalus and Anargyrolagus; Sánchez Vil-lagra et al. [2000]; Carlini et al. [2007]; Goin andAbello [2013]). The anatomical terms used for cra-nia and mandibles follow Wible (2003, 2007). Thepostcranial description is based on general ana-tomical contributions (Nomina Anatomica Veteri-naria; Schaller, 1992) and specific works relatedwith metatherian morphology (de Muizon, 1998;Szalay, 1994).

SYSTEMATIC PALEONTOLOGY

Infraclass MARSUPIALIA Illiger, 1811Order POLYDOLOPIMORPHIA Ameghino, 1897Suborder BONAPARTHERIIFORMES Goin and

Candela, 2004Superfamily ARGYROLAGOIDEA Ameghino, 1904

Family ARGYROLAGIDAE Ameghino, 1904Genus MICROTRAGULUS Ameghino, 1904

Type Species. Microtragulus argentinus Amegh-ino, 1904, by original designation.

Microtragulus bolivianus Hoffstetter and Villarroel, 1974

Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7

Holotype. MNHN-BOL-V-011707 (= GB-0001),right mandibular fragment with i1 and m3-m4 (GBrefers to the currently nonexistent GEOBOL [Servi-cio Geológico Boliviano] collection).Referred Material. JUY-P-0065, fragment of rightmaxilla with M3-M4; JUY-P 50, edentulous frag-ment of right maxilla preserving the zygomatic pro-cess; JUY-P-0066, right mandibular body with i1,alveoli of i2 and p3, and complete m1-4; JUY-P-0067, right mandibular body with complete i1 andi2, alveolus of p3, and complete m1-4; JUY-P-0068, anterior fragment of right mandibular bodywith i1 and alveoli of i2 and p3; JUY-P-0069,almost complete right humerus except for the prox-imal third; JUY-P-0070, almost complete lefthumerus except for the proximal third; JUY-P-0071, distal half of left humerus; JUY-P-0072, distalend of right humerus; JUY-P 59, proximal half ofright ulna; JUY-P 60, proximal half of left ulna; JUY-P 61, proximal fragment of radius; JUY-P 52, rightproximal epiphysis and right distal end of tibiofib-ula; JUY-P-0073, complete left calcaneus;JUY-P-

FIGURE 1. 1. Location map of the fossiliferous levels ofthe Uquía Formation exposed at San Roque, Huma-huaca (Jujuy Province, Argentina). Relative location ofthe prospected area in: 2, Jujuy Province; and 3, North-western Argentina. Modified from Ortiz et al. (2012).

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0074, body of left calcaneus; JUY-P-0075, rightastragalus; JUY-P 55, complete right metatarsal III;JUY-P 56, lot containing proximal and distal por-tions of right metatarsal IV (not associated); JUY-P57, distal portion of left metatarsal IV; JUY-P 58, lotcontaining three ungueal phalanges. Range and Occurrence. The holotype comesfrom Viscachani, 94 km SE of La Paz, Bolivia;Umala Formation (Pliocene levels dated between5.3–2.9 Ma [Marshall et al., 1992; MacFadden etal., 1994]). The materials studied herein come fromSan Roque (26º 14’ 32’’ S, 65º 21’ 55’’ W; 2940 m),4.4 km SSW of Humahuaca town, HumahuacaDepartment, Jujuy Province, Argentina; Uquía For-

mation (late Pliocene-early Pleistocene; MarplatanSALMA, probably Vorohuan subage; Ortiz et al.,2012). Emended Diagnosis. Microtragulus bolivianus dif-fers from Proargyrolagus by the presence ofeuhypsodont teeth, simplified occlusal structure,and reduced dental formula (which is also a differ-ence with Hondalagus and Anargyrolagus). Addi-tionally, M. bolivianus differs from the species ofHondalagus, Anargyrolagus, and Argyrolagus bythe following combined features: M3 subcircular inoutline and with a single labial cusp (metacone);M4 proportionally smaller than M3 and without dif-ferentiated cuspidal relief; lower molars with talonid

FIGURE 2. Line drawings (occlusal view) of Microtragulus bolivianus showing the anatomical nomenclature used inthe text. 1. Right M3-4. 2. Right m1-4 (image reflected). Abbreviations: cf, central fossa; cfd, central fossid; ectf, ectof-lexid; ectd, ectostylid; ent, entoconid; entd, entostylid?; entf, entoflexid; gr, groove; hyp, hypoconid; hypd, hypoconulid;mesl, mesiolabial lobe; met, metacone; metd, metaconid; metl, metaconule; prt, protocone; prtd, protoconid.

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proportionally shorter than the trigonid, reduced orabsent entoflexid; and proportionally smaller m4.Within Microtragulus, M. bolivianus differs from M.reigi by the absence of an entoflexid in m2, and ashallower entoflexid in m3; and from M. catama-rcensis by a proportionally larger m4 and the pres-ence of a well-marked entoflexid in m3.

Description

Maxilla. This bone is represented by two poorlypreserved fragments characterized by the pres-ence of several sulci, crests, and foramina (Figure3.1-3). A deep sulcus is present in the medial sideabove M4; this sulcus becomes progressively shal-lower towards the anterior side, and its trajectory

matches with the infraorbital canal for the passageof the maxillary nerve (a derivate of the maxillarybranch of the trigeminal nerve) and accompanyingvessels. This condition is inferred from the obser-vation of the specimen GHUNLPam 8549 (holo-type of Argyrolagus rusconii), which preserves theinfraorbital foramen connected to a medial sulcusof the maxilla. Other specimen referred to Micro-tragulus sp. (PVL 6594; García-López and Babot,2015), shows similar structures. Given thisarrangement, we infer that a proper infraorbitalcanal is absent, being replaced by a sulcus (corre-sponding to the medial wall of the canal) connectedanteriorly to a foramen piercing the maxilla anteriorto the orbit. This modified condition is probably

FIGURE 3. Microtragulus bolivianus. 1-3. JUY-P-0065, fragment of right maxilla with M3-M4 in: 1, lateral; 2, medial;and 3, occlusal views. 4-5. JUY-P-0066, right mandibular body with i1, alveoli of i2 and p3, and complete m1-4 in: 4,medial; and 5, lateral views. 6-7. JUY-P-0067, right mandibular body with complete i1 and i2, alveolus of p3, and com-plete m1-4 in: 6, medial; and 7, lateral views. 8-9. JUY-P-0068, anterior fragment of right mandibular body with i1 andalveoli of i2 and p3 in: 8, medial; and 9, lateral views. Scale bar equals 5 mm.

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related to the great development of the palatalvacuities typical of this family.

The specimen preserving the zygomatic pro-cess (JUY-P 50) probably corresponds to a juvenileindividual, judging by the posterior position of theprocess (above the alveolus of the last presentmolariform, which is very large). This would indi-cate that M4, which is the smallest molar, was notyet fully erupted at the time of death of the individ-ual.Mandible. The mandibular body is short and highand strongly convex ventrally. The point of maxi-mum height is located at the level of m2 (Figure3.4-7); in the labial side, it measures 5.65 mm inJUY-P-0067 and 4.94 mm in JUY-P-0066.

In lateral view the body exhibits the mentalforamina. In JUY-P-0066 there are three foramina,which are progressively smaller toward the poste-rior side; the first one is located at the level of p3.The second one is placed at the level of the mesial

border of m1, and the third opening is located atthe level of the mesial border of m2 (Figure 3.5).Only two foramina are visible in JUY-P-0067,although the surface where the third foramenshould be is actually broken (Figure 3.7). Theseopenings present the same location than the firstand second apertures in JUY-P-0066. Neverthe-less, the second foramen is proportionally smallerin this case. The specimen JUY-P-0068 (Figure3.9) shows a different arrangement: the two pre-served foramina open together inside a largeraperture (feature also present in Proargyrolagusbolivianus; Wolf, 1984). Other apertures are sev-eral tiny vascular openings scattered all over thelateral surface of the mandibular body.

The lateral side of the mandibular body alsopreserves a blunt but conspicuous coronoid crest.This structure projects anteroventrally towards thelevel of the anterior half of m4.

FIGURE 4. Microtragulus bolivianus, right lower dentition in occlusal view. 1. JUY-P-0066, detail of i1. 2. JUY-P-0066,m1-4. 3. JUY-P-0067, m1-4. Scale bar equals 5 mm.

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FIGURE 5. Microtragulus bolivianus, fore and hindlimb bones. 1-4. JUY-P-0069, right humerus almost completeexcept the proximal third in: 1, anterior; 2, posterior; 3, medial; and 4, lateral views. 5-8. JUY-P-0070, left humerusalmost complete except the proximal third in: 5, anterior; 6, posterior; 7, medial; and 8, lateral views. 9-12. JUY-P-0071, distal half of left humerus in: 9, anterior; 10, posterior; 11, medial; and 12, lateral views. 13-16. JUY-P-0072, dis-tal end of right humerus in 13, anterior; 14, posterior; 15, medial; and 16, lateral views. 17-19. JUY-P 59, proximal halfof right ulna in: 17, anterior; 18, lateral; and 19, medial views. 20-22. JUY-P 60, proximal half of left ulna in: 20, ante-rior; 21, lateral; and 22, medial views. 23. JUY-P 61, proximal fragment of radius. 24-25. JUY-P 52, distal end of righttibiofibula in: 24, anterior; and 25, posterior views. Scale bar equals 10 mm.


In medial view, the mandible shows anunfused symphysis, the borders of which are notwell defined. However, the surface is easily recog-nized by the presence of several shallow depres-sions and low tubercles. It extends to the level ofthe anterior portion of m2. Both specimens JUY-P-0066 and JUY-P-0067 bear several tiny nutritiousforamina and fenestrations towards the postero-ventral portion of the body (Figure 3.4, 6). Theangular process is partially preserved; its anteriorend is located at the level of the talonid of m4. Thevisible portion of this process indicates that it wasstrongly inflected forming a well-developed con-cavity facing upwards, as in other argyrolagids(Rusconi, 1933; Simpson, 1970a; Sánchez-Villagraet al., 2000). Two foramina are visible above theprocess: the dorsal one is the proper mandibularforamen and the aperture located immediatelybelow is interpreted here as an accessory openingof this canal. These secondary openings are alsopresent in the holotype of Microtragulus bolivianusas well as in other argyrolagids. The maxillarycanal (see Discussion) is not preserved in thespecimens recovered from Uquía Formation but isclearly present in the holotype of M. bolivianus(Hoffstetter and Villarroel, 1974).Dentition. The dentition is euhypsodont anddeeply implanted in the maxilla and dentary. The

teeth are prismatic, with central dentine sur-rounded by a layer of enamel. The teeth are alsopartially surrounded by a thin layer of cementumthat thickens in the area of the sulci. All the recov-ered dental pieces (except the only i2 of the sam-ple studied herein) show a central fossa/fossid.This structure is particularly conspicuous in i1,where the layer of enamel surrounding the innercavity is clearly present (Figure 4.1). This feature,described for Hondalagus (Villarroel and Marshall,1988), is also present in Argyrolagus (Simpson,1970b) and Microtragulus (García-López andBabot, 2015), and probably in AnargyrolagusMPEF-PV 5291.Upper dentition. M3 is sub-quadrangular (Figures2.1, 3.3) and larger than M4 (L=1.51 mm andW=1.41 mm). The intraalveolar portion of thecrown is slightly curved, vertically implanted, anddivergent with respect to the intraalveolar portion ofM4. The layer of enamel is continuous, although itis thinner in the mesiolabial area. The labial side isclearly higher than the lingual one, which is aboutthe same level of the alveolar border. In the labialedge there is a poorly developed mesiolabial lobeand a distal elevation that we interpret here as themetacone (metacone + StD after Goin and Abello,2013). The lingual side is rounded and bears theprotocone and the metaconule, which is very

FIGURE 6. Microtragulus bolivianus, astragalus and calcaneus. 1-5. JUY-P-0075, complete right astragalus in: 1,dorsal; 2, plantar; 3, medial; 4, lateral; and 5, distal views. 6-10. JUY-P-0073, complete left calcaneus in: 6, dorsal; 7,plantar; 8, medial; 9, lateral; and 10, distal views. Scale bar equals 5 mm.

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reduced and located slightly labially in relation tothe protocone. The central area of the occlusal sur-face bears a mesiodistally elongated and slightlyoblique fossa.

The M4 is simplified in relation to M3; it issmall (L=0.87 mm and W=0.71) and in occlusalview the crown is roughly triangular (Figures 2.1,3.3). The labial edge is convex and lacks flexi andcusps. Lingually, there are no proper cusps but thepresence of a protocone can be inferred, followingthe arrangement in the precedent molar. Themetaconule is absent. The enamel distribution isirregular; it is absent in the mesiolabial side of thetooth and very thin in the distolingual part. As inother Argyrolagidae, the occlusal surface of M4 ismesioventrally directed, contrasting with the dis-toventrally orientation of the preceding molar. Asubcircular fossa occupies a central position in theocclusal surface. Lower dentition. The i1 is kidney-shaped (thelabial border is convex and the lingual one is con-cave), labiolingually compressed, procumbent, anddeeply implanted (the intraalveolar section extendsup to the level of m1) (Figures 3.4-9, 4.1; Table 2).The tooth shows a continuous layer of enamel. Inlateral view, the occlusal surface is concave; themesial side is acute and high, and the distal one isblunt and low. It shows a small and oval fossid nearthe medial area of the lingual wall.

The second incisor is located immediatelybehind i1 (Figure 3.6-7). It is smaller and oval inoutline. Although hypsodont, this tooth is notdeeply implanted since the length of its alveolus isventrally limited by the alveolus of i1. In lateralview, the crown is mesially orientated. In contrastto i1, i2 is surrounded by a thick layer of cemen-tum. The occlusal surface presents a well-devel-oped wear facet facing labially.

The third premolar is not preserved in any ofthe specimens of Microtragulus bolivianus herestudied. Its alveolus is separated from i2 by a shortdiastema, whose mesiodistal length is about thesame length of the alveolus of i2. The alveolar sizeand shape suggest that this tooth was markedlysmaller than the first molar (as in other argyro-lagids) and not deeply implanted (Figure 4.2-3).

All the lower molars are euhypsodont. Theenamel distribution is mostly continuous aroundthe tooth, with small interruptions in the mesial anddistal edges of m1-3 (and a small portion of themesial edge of m4), and the ectoflexid of allmolars. Additionally, the layer of enamel of themesial side of m2 and m3 can be absent or be verythin. The cementum has an irregular arrangement.

FIGURE 7. Microtragulus bolivianus, metatarsal III andIV and phalanges. JUY-P 55, complete right metatarsalIII and JUY-P 56, distal portion of right metatarsal IV(not associated) in: 1, anterior; and 2, posterior views.Scale bar equals 10 mm. 3-5. JUY-P 58, ungual phalan-ges in lateral view. Scale bar equals 5 mm.

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In most cases, it is thicker in the deepest portion ofthe ectoflexid and in the labial wall of the talonid.The central fossid is mesiodistally elongated (Fig-ures 2, 4.2-3).

The first molar shows a distinct ectoflexid,which separates the trigonid from the talonid. Theentoflexid is absent in m1, as in Microtragulus cata-marcensis. This trait is present but vestigial in M.reigi (Simpson, 1970a) and clearly demarcated inall the other argyrolagids. The metaconid is thedominant cusp; it is a high crest-like structure mesi-ally displaced; slightly posterior, the lingual wall ofthe tooth exhibits a tiny bulge that could be anaccessory cusp (entostylid?; see Hershkovitz,1971). The ectostylid is lower and presents a well-developed column in the labial wall of the tooth.Mesially, a groove separates this structure from theprotoconid which points mesiolabially. The paraco-nid is absent. The talonid, two thirds shorter thanthe trigonid, is labiolingually extended with its mainaxis transversal to the tooth row. It has a distinctivetongue-like hypoconid and a well-developed ento-conid. The hypoconulid is a vestigial structurebarely distinctive in the distolingual corner of thetalonid and adjacent to the entoconid.

The second molar differs from the first one inits larger size, a shallower mesial groove in the tri-gonid (absent in JUY-P-0066), and a wider ectof-lexid. As in m1, the entoflexid is absent. Thehypoconid, entoconid, and hypoconulid have thesame development and arrangement than in m1.

The third molar is shorter than the precedingteeth but longer than m4. The trigonid is very simi-lar in outline to that of m2, although both in JUY-P-0066 and in JUY-P-0067 the groove mesial to theectostylid is absent. The talonid is well differenti-ated in this tooth, since a visible but shallow entof-lexid lingually separates these structures. Theectoflexid is very similar to that of m2. The entoco-nid is more conspicuous and the hypoconulid isabsent. The layer of enamel appears to be reducedin the mesial end of the trigonid in JUY-P-0066 andin the mesial and distolingual edge in JUY-P-0067.

The fourth molar is very different in shape andsize from the preceding molars. It is the smallestmolar, both in length and width, and the talonid islabiolingually reduced. As in m3, the trigonid isseparated from the talonid by the ecto and entof-lexids. The hypoconid is small and low, and theentoconid is a higher crest-like structure. The distalborder of the tooth bears a shallow flexid that sepa-rates the hypoconid from the distally extendedhypoconulid. This arrangement is evident in JUY-P-0067 but is masked in JUY-P-0066 given thepoor development of the hypoconulid. Postcranial Skeleton. Among argyrolagids, thepostcranium is known from partially preservedremains of Microtragulus argentinus (cuboid,navicular, ectocuneiform, metatarsals III and IV,and caudal vertebrae), M. reigi (humerus, femur,calcaneus, and tibiofibula), and Argyrolagus sca-gliai (partially preserved vertebrae, scapula,humerus, radius, ulna, pelvic girdle, femur, tibiofib-ula, tarsals, metatarsals, and phalanges). In addi-tion, an isolated calcaneus (not directly associatedto any Anargyrolagus specimen, but coming fromthe same level and locality; A. Carlini, personalcommun., 2016) was recovered at Gaiman, ChubutProvince and described by Szalay (1994). Up tonow, the anatomical descriptions came from thework of Simpson (1970a), which was focused on A.scagliai, M. reigi, and M. argentinus, and Szalay(1994) who emphasized on the morphology of theGaiman calcaneus and the crurotarsal joint and tar-sal anatomy of Argyrolagus. Here we present adetailed description of isolated pieces of the post-cranium of Microtragulus bolivianus, whichincludes partially preserved humeri, ulnae, radii,tibiofibulae, and complete astragalus, calcaneus,metatarsals, and distal phalanges. According toour observations, there are no significant differ-ences in the morphology of the skeletal piecesamong these species. However, in the followingsection we include several comparisons in order tohighlight inter and intraspecific variations.

TABLE 2. Measurements of the lower dentition of Microtragulus bolivianus (in mm). * Measures taken from Hoffstetterand Villarroel (1974).

i1 i2 m1 m2 m3 m4 m1-m4 i1-m4

L W L W L W L W L W L W L L

Holotype* 1.3 0.6 - - - - - - 1.5 1.3 1.4 1.0 - 10.3

JUY-P-0066 1.35 0.59 - - 1.50 1.14 1.53 1.49 1.44 1.34 1.14 1.08 5.77 11.01

JUY-P-0067 1.66 0.70 0.92 0.63 1.68 1.27 1.88 1.61 1.68 1.47 1.49 1.19 6.44 12.50

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Humerus. Four partially preserved humeri (two leftand two right) were recovered from the Uquía fossilassociation (Figure 5.1-16). These elements varyin size and robustness: JUY-P-0070 (Figure 5.5-8)is larger and stouter than the other three humeri, afeature that could be related to sexual or ontoge-netic variation. In anterior view the diaphysis isstraight. The proximal half of the shaft appears tobe twisted in relation to the distal portion althoughthe absence of the head prevents to evaluateclearly this feature. This half bears the deltoidcrest. In contrast to the general elongated shape ofthis structure in many mammals, in argyrolagids(e.g., Microtragulus bolivianus, Argyrolagus) theplace of attachment of the M. deltoideus and M.pectoralis major is a short, raised plate-like andovoid area facing anterolaterally. Laterally, the dis-tal half of the humerus bears a well-developed andhook-shaped ectepicondylar crest, which facesanterolaterally and ends proximally in an acute tip.Even if this crest is not fully preserved, it appearsto have a different arrangement in relation to Argy-rolagus where it does not flare laterally (Simpson,1970a). Medially, the distal portion of the diaphysisexhibits a large entepicondylar foramen in whichthe medial aperture is hidden in anterior view. Thesame morphology has been described by Simpson(1970a, p. 27) in Argyrolagus scagliai. The distalepiphysis exhibits a rounded capitulum in continu-ity with an ectepicondyle not expanded laterally.The trochlea is bounded medially by a sharp edgeseparating it from the entepicondyle. This structureis not medially expanded. The intercondylar notchis deep. Posteriorly, the supratrochlear foramenopens in the supratrochlear fossa which widensmedially. Several nutritious foramina pierce thehumerus. The medial wall of the entepicondylarforamen bears one (JUY-P-0069), two (JUY-0072),or three (JUY-P-0070, 0071) apertures. Otheropenings are visible in the proximal edge of theectepicondylar crest, adjacent to the diaphysis,where the specimens JUY-P-0069 and JUY-P-0070 bear one foramen. Finally, foramina are alsovisible above the posterior side of the supratroch-lear foramen (JUY-P-0070, 0071, and 0072 showone aperture and JUY-P-0069 shows two aper-tures).Ulna. Two incomplete ulnae were preserved (Fig-ure 5.17-22). The distal end is lost in both ele-ments. The olecranon is short and stout; theanteroposterior length is greater than the proxi-modistal length. The anconeal process faces later-ally and is not part of the anterior border of theolecranon which is well-projected anteriorly. The

trochlear notch is deep. The coronoid process isprominent and its articular surface is proximallydirected. Distally, the shaft exhibits a concave andtriangular area related to the insertion of the M.biceps brachii. The radial notch is concave, facingmore laterally, instead of anteriorly. It is well limitedposteriorly by an acuminate process. The body isincomplete; nonetheless, it is clear that it narroweddistally. The most conspicuous feature of the shaftis the thinness of the bone and the extremely con-cave fossa for the M. abductor pollicis longus. Thisfossa is extended along almost the entire shaft andis limited anteriorly by a sharp crest which is later-ally bent in anterior view. In medial view, the proxi-mal third of the ulna exhibits a shallow fossa for theinsertion of the M. flexor digitorum profundus.Radius. The proximal end of a radius is preserved(Figure 5.23). The incompleteness of this elementprevents us from determining if it is a right or a leftbone. The head is round and the fovea capitis isslightly concave. The neck is conspicuous; itshows a well-developed radial (bicipital) tuberosity,as in Argyrolagus (Simpson, 1970a). The body hasa deep posterior fossa probably related with theorigin of the M. abductor pollicis longus. Laterally,this fossa is bounded by a sharp pronator crestwhere the M. pronator teres would be inserted. Tibiofibula. This complex is represented by a rightproximal epiphysis and a fragment of a right distalthird. The proximal epiphysis is triangular in proxi-mal view. This area exhibits the medial condylewhich is subtriangular and slightly convex; the lat-eral one is kidney-shaped, concave, and longeranteroposteriorly. The popliteal notch is conspicu-ous and the intercondylar eminence exhibits boththe lateral and medial tuberosities (being the lateralremarkably smaller than the medial one). Theextensor sulcus is absent. Anteriorly, the materialpreserves part of the tibial tuberosity that is a smalland smooth surface.

The distal portion of the tibia and the fibula iscompletely fused (Figure 5.24-25); there is no evi-dence of sutures distinguishing neither this pair ofbones nor the diaphysis from the distal epiphysis.In anterior view this epiphysis exhibits a markedfossa, proximodistally elongated. In posterior view,the shaft has a blunt and oblique longitudinal crest.The tibia bears in its medial face a very shallownotch that indicates the position of the passage ofthe tendons of the muscles flexing the foot. In turn,the extensor muscles in the lateral side of the fibulapass through a deep groove (Szalay, 1994).

The joint between the tibiofibula and the tar-sus includes the articulation with the astragalus

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(medial) and with the calcaneus (lateral). As Sza-lay (1994) described for Argyrolagus, the crurotar-sal joint is highly restricted in Microtragulus; boththe fibular and tibial sides contact closely with theastragalus. In addition, the fibular portion jointswith a large area in the calcaneus. The astragalo-tibial facet is formed by two well-differentiatedmedial and lateral facets. In the same way themedial astragalotibial facet exhibits a horizontaland vertical surface. The horizontal one is concaveand anteroposteriorly elongated and the vertical isconvex and located in the lateral wall of the medialmalleolus. The lateral astragalotibial facet is veryconvex. On the fibula, the astragalofibular facet is asmall surface located in the medial side of the lat-eral malleolus. This facet is continuous with the lat-eral astragalotibial facet; therefore, both facets arehard to identify. The calcaneofibular facet is con-cave, oval, and anteroposteriorly elongated. Itsmain axis is shorter than the medial astragalotibialfacet. Both the morphology of the crurotarsal joint,as well as that of the tarsals is essentially the samein Argyrolagus and Microtragulus.Astragalus. The astragalar body (corpus tali) isremarkably large; it contrasts with a small head,which occupies one third of the total length of thedorsal face (Figure 6.1-5). In dorsal view the bodyexhibits the large astragalotibial lateral facet (ATil).It is well extended, both distally and proximally, andis somewhat pulley-like, being the medial halfsteeper than the lateral one. The astragalar fibularfacet is small and the astragalar foramen is absent.The head exhibits the dorsal portion of the astraga-lonavicular contact, which is proximolaterally-disto-medially extended. Medially, this facet shows theastragalar distal tuber. The proximal extension ofthe lateral tibial contact, the ectal (calcaneoastra-galar) facet, and the sustentacular and navicularfacets are visible in plantar view. The ectal facet isoval and strongly concave. The main axis isroughly orientated proximomedially-distolaterally.The sustentacular facet is ribbonlike and contactswith the homonymous facet in the calcaneus; itcomprises the sustentacular facet properly and thesuperior sustentacular facet. The first one isformed by two areas, one occupies the centralexposition of the plantar face and the other is rod-like and oblique. Both are tangential and its contactis located near the proximomedial end of the rod-like area. The central area of the sustentacularfacet is oval, slightly wider distally, and is almostparallel to the ectal facet. The rod-like portion of thesustentacular facet is well exposed in plantar view.It is remarkably convex, and occupies the full width

of the astragalar head. A deep and narrow astraga-lar sulcus (interarticular sulcus) separates thisfacet from the calcaneoastragalar articulation. Thesuperior sustentacular facet is represented as adeep sulcus between the plantar extension of theastragalotibial lateral facet and the sustentacularfacets. The astragalar distal tuber is small and isrestricted to the proximomedial portion of the plan-tar aspect. The astragalonavicular facet isrestricted to the distolateral extremity of the head.

In medial view the dominant structure is thesemicircular astragalotibial medial facet, clearlydefined by a blunt dorsal edge. On the proximaland plantar corner there is a small astragalarmedial plantar tuberosity located adjacent to thesuperior sustentacular facet. Distally this aspectexhibits the medial extension of the contact withthe navicular and the astragalar distal tuber. In lat-eral view the astragalus exhibits the astragalofibu-lar facet which is small, triangular, and faceslaterally. The distinctive feature of the distal view isthe astragalonavicular joint, which covers almostthe entire surface of the head; there is no articula-tion between the astragalus and the cuboid.Calcaneus. The tuber is a cylindrical lateromedi-ally compressed structure which widens toward itsposterior end (Figure 6.6-10). This end exhibits twosmooth surfaces separated by a low step; one ismainly posterior and the other is more plantarlydirected. In lateroplantar view the tuber shows anoval scar which was probably related to with theattachment of the M. abductor digiti quinti. A bluntcrest runs anteroposteriorly in plantar view andends in a small anterior plantar tubercle. The pero-neal process is located in the laterodistal corner ofthe plantar aspect. This process is rounded andwell developed. Adjacent to the peroneal process,and distal to it, there is a small groove for the M.peroneus longus.

The body of the calcaneus is wide and muchshorter than the tuber. In dorsal view the bodyshows the proximal process bearing the calcaneo-astragalar (or ectal) facet and the calcaneofibularfacet. Both facets are separated by a shallowgroove in Argyrolagus and Microtragulus, differingfrom the Gaiman calcaneus in which they are closetogether (see Szalay, 1994, figure 7.28 A, C). Thecalcaneoastragalar facet faces dorsally and dorso-medially and is smaller and more dorsally pro-jected than the calcaneofibular facet (particularlyvisible in medial and distal views). Both in Argyrola-gus as in Microtragulus, the calcaneoastragalarfacet is smaller than that of the Miocene Gaimancalcaneus (Szalay, 1994). The sustentacular facet

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comprises the sustentacular facet properly and thesuperior sustentacular articulation. As in theastragalus, the sustentacular is divided in two por-tions: one is proximal, oval, and distally orientedand the other is more distally located, smaller, andfaces dorsomedially. The superior sustentacularfacet is a small articular surface restricted to thedorsal border of the proximal part of the sustentac-ular facet. In distal view, this facet is smaller inArgyrolagus than in the Gaiman calcaneus (Szalay,1994), a condition also present in Microtragulus.

The contact with the cuboid is formed by twodiscontinuous facets separated by a step. Theproximal facet (CaCup) is smaller and somewhattriangular; the distal one (CaCud) is dorsoplantarlyelongated and oval. Metatarsals. One complete right Mt III and threefragments of the Mt IV were recovered (Figure 7.1-2). The Mt III measures 29.87 mm, slightly largerthan Microtragulus argentinus (27 mm; Ameghino,1904) and smaller than Argyrolagus scagliai (35.6mm; Simpson, 1970a, p. 68). The Mt III exhibits aslightly concave T-shaped facet for the articulationwith the ectocuneiform. Laterally a marked notchlodges a medial process for the Mt IV; medially,there is a distinctive facet which is interpreted asthe contact with a vestigial Mt II, as proposed forArgyrolagus (Simpson, 1970a, figure 15E). Theproximal end of the Mt IV exhibits a triangular con-vex surface which articulates with the cuboid. Lat-erally this bone presents a proximally projectedprocess that probably covered partially the lateralside of the cuboid, as in Microtragulus argentinusMACN 12925 and Argyrolagus scagliai MMP 785-S (Simpson, 1970a). The only peculiarity of thediaphysis of the metatarsals is the flattened area ofcontact between Mt III and IV. In this last bone, thedistal half of the diaphysis is thinner than in Mt III.The distal end of the metatarsals is simple. In thedorsal aspect of the head the articular surface iscondylar; in plantar view this structure shows anotorious longitudinal crest. Phalanges. Three ungual phalanges were recov-ered (Figure 7.3-5). They are long, slightly curved,and lateromedially compressed. The flexor tuber-cle is long, constricted in the middle portion, and ispierced by the ungual foramen. The articular facetis strongly concave; the dorsal border is more pos-teriorly projected than the ventral one.

Comparison

When compared with Oligocene and Mioceneargyrolagids (Proargyrolagus, Hondalagus, andAnargyrolagus), Microtragulus bolivianus shows

clear differences involving several features.Regarding Proargyrolagus, the main dental differ-ences include the degree of hypsodonty, the mor-phology of the occlusal surface, and the dentalformula. In Proargyrolagus the molars are hypsod-ont but rooted (i.e., high crown, not evergrowing;Wolf, 1984; protohypsodont in Goin and Abello,2013) while in M. bolivianus the molars are euhyp-sodont (i.e., high crown and rootless; Hoffstetterand Villarroel, 1974). The occlusal surface of theupper molars in Proargyrolagus exhibits a complexoutline including several distinctive cups and struc-tures (e.g., mesiolabial lobe, paracone + StB,metacone + StD, protocone, and metaconule). InM. bolivianus this pattern is simplified with most ofthe occlusal features largely masked. Another dis-tinctive feature of M. bolivianus regarding Proargy-rolagus is related to the crown implantation in M3-4. In Proargyrolagus the crowns of both molarspresent an almost parallel implantation, while in M.bolivianus the intraalveolar part of the crowns isstrongly divergent. This divergence determinesthat the occlusal surface of M3 faces ventrally andthat of M4 faces mesioventrally. These differencesin implantation are also visible in the lower molars.Additionally, M. bolivianus also lacks several dentalpieces compared to Proargyrolagus (dental for-mula 2.0.1.4 for M. bolivianus and 3.1.2.4 or4.0.2.4 for P. bolivianus; see Sánchez-Villagra andKay [1997] for different interpretations on the lowerdental formula in the Oligocene species). Finally,the euhypsodont nature of the lower molars in M.bolivianus also determines that the height of themandibular body is greater in this species.

The upper teeth of the Miocene genus Honda-lagus differ mainly in size (smaller in relation toMicrotragulus bolivianus), outline (more trans-verse in Hondalagus), and the distinctive layer ofcementum (Sánchez-Villagra et al., 2000), which ismuch thinner in M. bolivianus. Hondalagus has anextra anterior lower tooth (Sánchez-Villagra et al.,2000), a well-developed entoflexid in m1 and m2(absent in M. bolivianus), and proportionally largertalonids (particularly in m4).

The dental formula is the main differencebetween Anargyrolagus and Microtragulus bolivi-anus, since the former presents a canine and twomore premolars than Microtragulus. In Anargyrola-gus M3 is more quadrangular, with a well-devel-oped metaconule, and M4 shows a markedocclusal relief in contrast with M. bolivianus, whichpresents an almost flat occlusal surface. The firstlower incisors show a clear fossettid near themedial area of the lingual wall in M. bolivianus,

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absent in Anargyrolagus. This genus also contrastswith M. bolivianus by the higher mandibular body,the well-developed entoflexid and proportionallylarger talonid in all the molars, and a proportionallylarger m4.

Regarding the genus Argyrolagus, whichincludes four species (A. scagliai, A. parodii, A.palmeri, and A. rusconii), the upper dentition will becompared only with A. scagliai and A. rusconiisince A. parodii and A. palmeri are representedonly by lower teeth. These two species show M3longer than wide, with the paracone present andwell separated from the metacone, the occlusaloutline with more distinctive corners and well-defined angles (especially in the protocone area),and the mesial border much wider than the distalone; all these features represent clear differencesregarding M. bolivianus. Additionally, the labial wallin both species is straight in M3, contrasting withthe convex wall of M. bolivianus. Regarding M4, inA. scagliai and A. rusconii this molar is proportion-ally larger and both the paracone and metaconeare distinctive. Finally, the lower molars of all thespecies of Argyrolagus are characterized by a visi-ble entoflexid (in m1-m4) and a larger talonid in allmolars, particularly in m4.

The new material of Microtragulus bolivianusdescribed herein can be compared more preciselywith the specimens known for M. reigi and M. cata-marcensis. In M. reigi, M3 is longer than wide,while in M. bolivianus the ratio between the lengthand the width is approximately one. Moreover, M3in M. reigi has a small paracone and sharp corners(i.e., a more angled outline). Additionally, the flexusbetween the mesiolabial lobe and the metacone,present in M. bolivianus, is absent in M. reigi; themesial border of the tooth is wider than the distal

one; and the layer of cementum is thicker in M.reigi. The relative size and shape of M4 is similarbetween these species, but the presence of differ-entiated paracone and metacone in M. reigi is notobserved in M. bolivianus. In the lower dentition,the morphology of the incisors is similar in bothspecies. The molars differ in the presence of asmall but distinctive entoflexid in m2-3 in M. reigi,and a more reduced talonid in its m4 which alsolacks the distal flexid distinguishable in M. bolivi-anus.

Microtragulus catamarcensis is smaller thanM. bolivianus, and the diastema between i2 and p3is relatively shorter. Additionally, the talonid of m3lacks the entoflexid, clearly present in M. bolivi-anus. The talonid in m4 has a simpler morphologyin M. catamarcensis: the cusps are indistinguish-able and the distal flexid present in M. bolivianus isabsent.

Microtragulus sp. Ameghino, 1904Figure 8

Referred Material. JUY-P 53, isolated left M3;JUY-P 54, isolated left m2 or m3.Range and Occurrence. San Roque (26º 14’ 32’’S and 65º 21’ 55’’ W; 2940 m), 4.4 km SSW ofHumahuaca town, Humahuaca Department, JujuyProvince, Argentina. Uquía Formation (late Plio-cene-early Pleistocene; Marplatan SALMA, proba-bly Vorohuan subage; Ortiz et al., 2012).Remarks. The isolated upper molar (Figure 8.1) isconsidered herein an M3. The tooth is subquadran-gular and exhibits an irregular central fossa; themesiolabial border is marked as an angled corner.The area of the metacone is represented by asmooth elevation and the area of the protocone isslightly more differentiated (as a subtle corner)

FIGURE 8. Microtragulus sp., upper and lower molars in occlusal views. 1. JUY-P 53, isolated left M3. 2. JUY-P 54,isolated left m2 or m3. Scale bar equals 2 mm.

15


than the metaconular zone. The distribution of theenamel is irregular: it is clearly present in the labialwall of the tooth but is absent in the mesial side.The layer of cementum is clearly visible in the lin-gual and mesiolingual walls. The lower molar (Fig-ure 8.2) is characterized by a marked reduction ofthe talonid, both in length as in width. This condi-tion is similar in Microtragulus catamarcensisalthough in this species the hypoconid is U-shapedwhile in the isolated molar it is V-shaped. As inother members of Microtragulus (e.g., M. catama-rcensis and m1-2 of M. bolivianus) the entoflexid isabsent. The central fossid has the usual shape forArgyrolagidae, which is mesiodistally elongated.

We discard the inclusion of these molars inMicrotragulus bolivianus based on their smallersize, the general outline of the upper molar (subcir-cular in M. bolivianus vs. quadrangular in JUY-P53), the absence of a flexus between the mesiola-bial corner and the metacone, and the angledhypoconid in the lower tooth. Nevertheless, the iso-lated nature and scarcity of this material prevent usfrom providing a specific designation.

DISCUSSION

On the Nomenclatural Problems Related to the Family Argyrolagidae and the Genera Microtragulus and Argyrolagus

Since the erection of the genus and speciesMicrotragulus argentinus by Ameghino (1904)based on postcranial elements, the name of thefamily Argyrolagidae and the genera Microtragulusand Argyrolagus have been through extensivenomenclatural changes (see Appendix).

The basis of the conflict is that the type andonly specimen of Microtragulus argentinus wasoriginally included in the Family Tragulidae (Cetar-tiodactyla) and is represented only by postcranialelements (cuboid, navicular, and ectocuneiform,metatarsals III and IV, and two anterior caudal ver-tebrae) which lack diagnostic features (Hoffstetterand Villarroel, 1974). In a latter issue of the samevolume, Ameghino (1904) erected the Family Argy-rolagidae, which he included in Lagomorpha, andthe species Argyrolagus palmeri, represented by aleft mandibular fragment with m1-m4. This materialcomes from the same locality and formation thanM. argentinus (Monte Hermoso Formation, MonteHermoso locality, Buenos Aires), although thestratigraphical correspondence cannot be ascer-tained since these data remain unknown for theholotypes of M. argentinus and A. palmeri. Theprobability that the types of M. argentinus and A.

palmeri pertain to the same species was initiallynoted by Carlos Ameghino and Rusconi (see Rus-coni, 1936, p. 181).

After the work of Ameghino, some authorsfocused on this nomenclatural issue and sug-gested the synonymy and priority of Microtragulusover Argyrolagus (Rusconi, 1936) and evenattempted to establish the use of Microtragulidaeas the valid name for the family (Reig, 1955). Nev-ertheless, it was not until 1970 when these succes-sive modifications were extensively andsystematically reviewed by Simpson. This authorproposed a conservative and arbitrary, thoughpractical, taxonomic scheme. Based on a morecomplete specimen collection, he found two cra-nial/dental morphotypes varying in form and size.The larger morphotype included the Argyrolagusspecies (A. palmeri, A. parodii, and the then new A.scagliai) and, since the metatarsals assigned to M.argentinus are 25% smaller than those of A. sca-gliai, Simpson decided to maintain the name Micro-tragulus and associate the smaller toothmorphotypes (M. catamarcensis and the then newM. reigi) to this genus. The association wasexplained by the similar estimated metatarsallength of M. reigi (based on the metatarsal lenght/m1-m4 length ratio of A. scagliai), and the knownmeasurement of M. argentinus. This proposal waswidely accepted, although some authors continuedusing the name Microtragulidae (e.g., Tonni et al.,1992; Cione and Tonni, 1995; Vizcaíno et al.,2004).

By the time of his contribution, Simpson sus-pected that the only way to resolve the possiblesynonymy between the two Monte Hermoso gen-era was to recover associated metatarsal bones“clearly referable to Microtragulus argentinus and amandible of the same individual” (Simpson, 1970a,p. 4-5). However, we now realize that this factwould be hardly the solution. As is shown in thetaxonomic description above, with the exception ofsize, there is almost no morphological variationamong the comparable postcranial elements of M.bolivianus and M. argentinus (i.e., metatarsal andtarsal bones), and M. bolivianus and Argyrolagusscagliai. Moreover, the size differences can beobviously associated to intraspecific variability, ashas been reported for other species of similar sizeand ecological requirements (see Nader, 1978).

In this sense, we support the idea that themetatarsals referred to Microtragulus argentinuscorrespond to Argyrolagus palmeri and agree withthe following arguments previously proposed byHoffstetter and Villarroel (1974, p. 1947): a) the

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specimens referred to these taxa are the onlypieces coming from the Monte Hermoso clefts; b)the metatarsal bones do not exhibit intraspecificmorphological variation among argyrolagids; andc) the metatarsal length known and estimated forthe metatarsals of M. argentinus and A. palmerican be explained by intraspecific variation. In rela-tion to argument a), the Monte Hermoso fauna iscurrently one of the best studied fauna of Argen-tina. In the last 50 years, extensive fieldwork anddifferent studies have focused on the resolution ofthis mammal age, including its mammal diversity,age, and sedimentology (Cione and Tonni, 1995,2005; Deschamps et al., 2012; Tomassini et al.,2013 and literature therein). In this lapse, two addi-tional specimens were found and referred to A.palmeri (Tomassini et al., 2013) and no specimensof a smaller morphotype referable to Microtraguluswere found.

At this point, the two conflicts derived from thiscontext are the validity of the name Argyrolagusand thus, the use of Argyrolagidae or Microtraguli-dae as the family name. The decision to maintainArgyrolagidae (and thus, Argyrolagus as typegenus for the family) is supported by Article 64 ofthe International Code of Zoological Nomenclature(ICZN, 1999). Nevertheless, this fact would only bepossible if the synonymy between Microtragulusand Argyrolagus is discarded and both genericnames considered valid. This is, according toSimpson (1970a) and in our view, the most recom-mended scenario, since any other option wouldrequire more complicated, controversial, andhardly acceptable actions.

Anatomical Singularities of the Dentary of Argyrolagids

The dentary of argyrolagids is characterizedby a high mandibular body, the presence of two tothree dental foramina, an unfused mandibular sym-physis extended to the level of m1 or m2, the pres-ence of small pits and fenestrations mainly locatedin the posterior half of the medial side of the den-tary, a robust and anteroventrally projected coro-noid crest, a low and short coronoid processbearing a masseteric foramen, a well-expandedposterior shelf of the masseteric fossa and angularprocess, a low condyle placed above the tooth row,and other peculiar features such as a markedlylong mandibular notch (ventral end of the posteriorborder of the coronoid process), and the presenceof retromolar foramina and a maxillary canal (Rus-coni, 1933, 1936; Simpson, 1970a; Hoffstetter andVillarroel, 1974; Sanchez-Villagra et al., 2000, San-

chez-Villagra, 2001; Goin and Abello, 2013). Someof these traits exhibit intrageneric variation amongthe basal Deseadan, Miocene, and Pliocene taxa.In Proargyrolagus bolivianus the mandibular bodyis lower than in the more recent types (Wolf, 1984;Sánchez-Villagra and Kay, 1997), probably inassociation with the protohypsodont condition ofthe molars (Wolf, 1984). The coronoid crest is lon-ger in Proargyrolagus and Hondalagus, almostreaching the ventral border of the mandible (judg-ing by the images from Sánchez-Villagra et al.,2000, figure 4a, c). In Anargyrolagus, Argyrolagus,and Microtragulus this crest shortens dorsoven-trally but projects more anteriorly, towards the levelof the limit between m3-m4. There is also variationamong these taxa; for example, in Argyrolagusparodii it is stronger than in M. bolivianus.

Posterior and dorsal to the retromolar spacethe dentary of argyrolagids exhibits some notice-able traits. At the level of the alveolar border thecoronoid crest expands and forms a lateral lamina(as described by Simpson, 1970a), which limits theretromolar fossa. Small foramina open in the pos-terior end of this concavity, clearly visible in Anar-gyrolagus (Goin and Abello, 2013, figure 4.17),Argyrolagus scagliai (Simpson, 1970a), A. palmeri(Kraglievich, 1931), A. parodii (Rusconi, 1933), andMicrotragulus bolivianus (Hoffstetter and Villarroel,1974; this paper). These openings, called the retro-molar foramina, are also present in some groups ofmammals such as abderitids (Rusconi, 1933;Abello and Rubilar-Rogers, 2012, figure 6.4) andhumans (Schejtman et al., 1967; Kumar Potu et al.,2014). In modern mammals the retromolar foram-ina are related with the mandibular canal that car-ries the inferior alveolar nerve and vessels(Schejtman et al., 1967). Other foramina, probablyrelated with this internal net of canals, are presentat the base of the medial and lateral sides of themandibular ramus.

Another particular trait of the argyrolagid man-dible is the presence of a short medially locatedcanal at the base of the coronoid process and pos-terior to m4 (Figure 9). This canal is bounded medi-ally by a bridge-like vertical osseous bar andlaterally by the mandibular ramus. This structurewas observed in Microtragulus bolivianus by Hoff-stetter and Villarroel (1974) and named the retro-dental canal. In modern mammals (e.g.,Lagomorpha) it is called the maxillary canal and isthe passage of a vein that connects the inferioralveolar and inferior orbital veins (Wible, 2007).Sánchez-Villagra et al. (2000) misinterpreted thistrait as a common feature of Argyrolagidae and the

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18

FIGURE 9. Argyrolagus sp. MACN 17590, left mandibular fragment with m1-4. 1-4. Photograph and line drawing in:1-2, medial; and 3-4, dorsal views. The arrow points the maxillary canal. Scale bar equals 5 mm.


cenolestid Lestoros but in this genus, as well as inother caenolestids, the canal is actually absent.Therefore, among South American extant andextinct metatherians, the maxillary canal is aunique and singular feature of argyrolagids.

CONCLUSIONS

This contribution adds new materials to thehypodigm of Microtragulus bolivianus, expandingon several craniomandibular, dental, and postcra-nial remains. The dental distinctive features of thisspecies include M3 subcircular in outline and with aflexus between the mesiolabial lobe and themetacone (single labial cusp), M4 proportionallysmaller than M3 and without differentiated cusprelief, lower molars with talonid proportionallyshorter than the trigonid, entoflexid absent in m1-m2 and present but shallow in m3, and talonid inthe m4 proportionally large and with a distinguish-able distal flexid.

The cranial and mandibular morphology hereevaluated is fairly homogeneous among the Plio-cene genera. The mandibular body is short andhigh, and strongly convex ventrally. Many smallvascular openings are scattered over the lateraland medial surface of the body. The number, size,and arrangement of the mental foramina varyamong the observed specimens; two to threeforamina are variably located in the anterior half ofthe dentary. The coronoid process is low and short,and the coronoid crest is conspicuous and projectsanteroventrally towards the level of the anterior halfof m4. The mandibular condyle is low, placedabove the tooth row. A very peculiar feature of theanatomy of the dentary of argyrolagids is the maxil-lary canal, a structure probable related with thepassage of the vein that connected the inferioralveolar and inferior orbital veins and probableanalogue to a similar opening found in living Lago-morpha.

After reviewing all available specimens duringthis study we found the postcranium of argyro-lagids to be fairly uniform among members of thefamily (e.g., Argyrolagus, Microtragulus argentinus,M. reigi). Therefore, several skeletal features arenot useful enough for taxonomic or phylogeneticstudies among Argyrolagidae. This is relevantbecause the first species included in the family isM. argentinus, a taxon founded on postcranialremains. In part because of this situation, the prob-lems related to the validity of the names Argyrolagi-dae and Microtragulus were questionedrepeatedly. At present, this nomenclatural problemhas not been formally solved but a reasonable and

practical proposal was provided by Simpson(1970a). This option is here considered as themost adequate, and hence, we argue in favor ofmaintaining the names Argyrolagidae, Microtragu-lus, and Argyrolagus.

ACKNOWLEDGMENTS

We thank A. Kramarz and S. Álvarez (MACN),A. Dondas (MMP), M. Reguero (MLP), and A.Abello for access to the collections under theircare. Two anonymous reviewers and the editormade valuable comments that greatly improved thepaper. The present contribution was supported bythe Fundación Miguel Lillo and Facultad de Cien-cias Naturales e Instituto Miguel Lillo (UniversidadNacional de Tucumán, Argentina) and the Agenciade Promoción Científica y Tecnológica (FONCyT;PICT 0407).

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APPENDIX

Revision of the changes that occurred in the last century in the nomenclature of the Family Argy-rolagidae and the genera Microtragulus and Argyrolagus.

Author and year of publication and erected family and species or used names

Comments

Ameghino, 1904Family TragulidaeMicrotragulus argentinus

This taxon was founded on the basis of metatarsal and tarsal bones

Ameghino, 1904 Family ArgyrolagidaeArgyrolagus palmeri

A. palmeri is the type species of the genus Argyrolagus. Microtragulus argentinus and A. palmeri were published in the same year, but Microtragulus has priority because was published in an earlier issue of the same volume

Kraglievich, 1931Family ArgyrolagidaeMicrotragulus catamarcensis

This author named a small dentary coming from Catamarca Province as Argyrolagus catamarcensis. Posteriorly Simpson (1970a) proposed the new combination Microtragulus catamarcensis

Rusconi, 1936 Family ArgyrolagidaeMicrotragulus = Argyrolagus

This author suggested the synonymy but he did not consider the priority of Microtragulus under Argyrolagus and continued the use of Argyrolagus and Argyrolagidae (See also Rusconi, 1967)

Reig, 1955, 1958 Family MicrotragulidaeMicrotragulus = Argyrolagus

This author established this synonymy after he and Kraglievich studied the then new specimen referred as Microtragulus sp. (probably the holotype of Argyrolagus scagliai) and began the use of Microtragulidae with Microtragulus as the only valid genus

Ringuelet, 1966 Family MicrotragulidaeMicrotragulus = ArgyrolagusM. argentinus = A. palmeri

This author established the priority of Microtraguidae over Argyrolagidae, Microtragulus under Argyrolagus, and M. argentinus under A. palmeri. The material there figured (but not catalogued; see p. 58) was referred as M. argentinus but later assigned to the neotype of A. parodii (Simpson, 1970b) and catalogued as MLP 62-VII-27-81

Simpson, 1970aFamily ArgyrolagidaeMicrotragulus (including M. argentinus, M. catamarcensis, M. reigi)

This author made the first comprehensive attempt in order to resolve the nomenclatural problems in the family (see text for a complete explanation)

Hoffstetter and Villarroel, 1974Family MicrotragulidaeMicrotragulus (including M. reigi, M. catamarcensis, and M. bolivianus

They validated the genus Microtragulus (including Argyrolagus) and proposed tentatively the erection of the subgenus Microtragulus and Argyrolagus. Microtragulus bolivianusis presented as Microtragulus (Argyrolagus?) bolivianus in the original publication. The authors did not formalize the taxonomic status of the genera included in Argyrolagus

Tonni et al., 1992 Family MicrotragulidaeArgyrolagus palmeri

These authors included these taxon names in a taxomic list of the mammals recovered at Monte Hermoso (Buenos Aires) and included in the Montehermosan SALMA. No explanation was given in relation to the exclusion of M. argentinus from this list

Cione and Tonni, 1995 Family MicrotragulidaeMicrotragulus (including M. reigi)Argyrolagus (including A. palmeri, A. scagliai)

M. argentinus is not included among the taxa recovered at Monte Hermoso, Buenos Aires

Goin, 1995 Family ArgyrolagidaeMicrotragulus (including M. argentinus, M. reigi)Argyrolagus (including A. palmeri, A. scagliai)

The author followed the proposal of Simpson (1970a)

Vizcaíno et al., 2004Family MicrotragulidaeMicrotragulus reigiArgyrolagus scagliai

The authors followed Tonni et al. (1992)

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Sanchez-Villagra, 2001; Goin et al., 2000; Zimicz, 2011; Ortiz et al., 2012; Goin and Abello, 2013; Tomassini et al., 2013; García López and Babot, 2015. Family ArgyrolagidaeMicrotragulus (including M. argentinus, M. reigi¸ M. bolivianus)Argyrolagus (including A. palmeri, A. scagliai)

These authors followed the proposal of Simpson (1970a)

Author and year of publication and erected family and species or used names

Comments

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